A conventional local oscillation frequency synthesizer comprises a reference frequency generator for generating a reference frequency, a voltage controlled oscillator for generating a frequency dependent on a voltage applied thereto, a frequency divider for dividing the frequency of the voltage controlled oscillator by a predetermined number, a phase comparator for comparing phases of the reference frequency and the divided frequency, a charge pump for generating first and second voltages dependent on a phase difference in the phase comaprator, and a loop filter for applying an integrated value of the first and second voltages to the voltage controlled oscillator.
In operation, the reference frequency generator generates a reference frequency suited for a selected channel, and the reference frequency and a divided frequency supplied from the frequency division circuit are compared in phase by the phase comparator, so that a phase difference signal is generated therein. Then, the charge pump generates a voltage dependent on the phase difference signal, and the voltage is integrated in the loop filter, so that an integrated voltage is applied to the voltage controlled oscillator, from which a frequency dependent on the integrated voltage is supplied to the frequency division circuit. Consequently, a controlled carrier frequency is obtained in the voltage controlled oscillator.
In this conventional local oscillation synthesizer, capacitors are connected to outputs of the phase comparator to result in the expansion of the dead zone for phase comparison thereby improving a C/N value. In other words, a carrier frequency is changed in high speed, in the case where a gain of the loop system is high. For this purpose, the parameters such as a frequency division ratio, a property of the loop filter, etc. should be selected to achieve the above-noted high speed carrier frequency change in the loop system having high gain. When the gain is heightened in accordance with the property of the loop filter, a noise band becomes wide, so that the sensitivity of the phase difference signal becomes high, while noise tends to be introduced into the loop system. The introduction of noise into the loop system causes deterioration of a C/N The deterioration of the C/N value is conventionally overcome by providing the aforementioned capacitors.
In the conventional local oscillation synthesizer, however, there is a disadvantage in that a carrier frequency fluctuates up and down which results in the tailing-of vibrations in the vicinity of a converging value, because the dead zone for phase comparison is expanded.